Navigation systems have become integral components in today's automobile and transportation industries. Among other things, these systems can be used to provide a map of a current position, provide detailed driving instructions, identify nearby restaurants, and identify nearby points of interest. State of the art navigations systems sometimes combine Global Navigation Satellite System (GNSS), including Global Positioning System (GPS), Galileo, Compass and/or GLONASS techniques, with map matching techniques to achieve exceptional horizontal accuracy (i.e., current position with respect to longitude and latitude coordinates). Unfortunately, the inventors have recognized that presently available navigation systems at times encounter difficulties in providing accurate or reliable vertical accuracy (i.e., altitude).
The inventors have perceived that vertical accuracy in navigation systems is limited in part due to poor geometric dilution of precision (GDOP), which is caused when the satellites used to obtain GPS coordinates are clustered too close together. The inventors have further perceived that vertical accuracy is limited in urban or other areas in which direct reception of satellite transmissions becomes blocked. Buildings and other tall structures in urban areas can also cause an effect known as multipath, which further reduces the accuracy of navigation systems. Multipath can refer to the reception of a non-line of sight (i.e., reflected/refracted) signal from a satellite transmitter. Because the non-line of sight signal is reflected or refracted, it takes longer to reach the navigation system than would a signal received in the line of sight. As a result, the navigation system calculates (based on the fact that the transmission time of a signal is directly correlated to the distance traveled by the signal) that the vehicle is at a lower altitude (i.e., farther away from the satellite) than it really is.
The inventors have recognized that the vertical accuracy of a navigation system can be extremely important when a user is traveling on multi-level roads in which a first road is directly on top of a second road. In such multi-level roads, the lack of vertical accuracy makes it difficult for navigation systems to accurately distinguish between exits on the upper road and exits on the lower road. In multi-level roads with two-way traffic on each level, the lack of vertical accuracy also makes it difficult for navigation systems to distinguish between northbound/southbound (or eastbound/westbound) exits on the same level. As a result, timely notification of approaching exits is not provided to the user. Similar problems occur in ‘clover leaf’ freeway configurations in which two roads separate in almost parallel directions, but with different slopes.
Thus, the inventors have perceived a need for a navigation system which is capable of distinguishing between upper and lower levels of a multi-level road such that more accurate driving directions can be provided. Further, the inventors perceive a need for a navigation system which can provide accurate vertical measurements in an urban or other area in which the reception of multipath is problematic.